The present invention relates to a distributed feedback semiconductor laser (hereinafter referred to as a "DFB laser") having integrated therewith a photodiode equipped with a monitor function.
Since the DFB laser includes a corrugation grating of excellent wavelength selectivity, it is capable of implementing a single wavelength oscillation and is now under study for development as a light source for low-loss optical fiber communication. Generally, in case of employing a semiconductor laser, the magnitude of its optical output is monitored by a photodiode for the purpose of holding the output constant. It is customary, in this case, that the semiconductor laser and the photodiode are respectively formed by independent elements. Of semiconductor lasers, the DFB laser is not only capable of the single wavelength operation but also dispenses with a reflecting end facet which is indispensable to the conventional semiconductor laser, such as the cleavage plane; therefore, the DFB laser is suitable for monolithic integration with an element which has a function other than the laser oscillation. In particular, the afore-mentioned photodiode for monitoring the optical output can easily be integrated with the DFB laser.
However, the conventional DFB laser with monitor has a defect in that pn junctions of its laser region and its monitor region cannot be electrically separated by diffusion of zinc alone.